oils

In today's Curious Cook column I write about what happens to the flavors of cooking oils when they're heated, and about the value of paying attention to oil flavor. It turns out that delicious and pricey extra-virgin olive oils lose much of their aroma in the pan. And stale, rancid oils are less healthful than fresh oils. I also mention the dismaying finding of researchers at the University of California at Davis that many American consumers prefer the flavors of defective olive oils to the flavors of high-quality oils--probably because they seldom get the chance to taste good oils!

Due to an editing error, the print edition of the column misstates the identity of the taste panel that tested the set of olive oils before and after cooking. It was the research panel of the University of California Cooperative Extension program, not a taste panel at U.C. Davis or its Olive Center. The web edition of the column has been corrected.

Here are a couple of interesting details to add to the story of olive oil's back-of-the-throat, peppery pungency.

In 2003, a research group at Unilever reported on the sensory effects of several of the phenolic compounds found in olive oil, including the cough-inducing substance later named oleocanthal. Oleuropein, the phenolic compound that makes the fruit of the olive bitter, is water-soluble rather than fat-soluble, so it doesn't get transferred into the oil when the fruit is pressed. But a number of oleuropein relatives do end up in the oil, and the Unilever researchers tasted them in pure form. They found these compounds to contribute mainly bitterness and drying astringency, along with some numbing, cooling, sour, salty, and tingling sensations. Relatives of pungent oleocanthal also taste somewhat astringent and bitter. So an oil rich in phenolics can have a very complex taste and mouthfeel indeed.

Other studies have found that heating olive oil reduces the levels of most phenolic compounds. And pepperiness gradually fades as an oil ages. The balance of flavors in an excellent olive oil is thus temporary and so especially worth savoring when you find it. The deterioration of olive oil is slowed by keeping it cool and protected from light.

Refrigeration usually slows the deterioration of our foods by slowing the chemical reactions that cause it. But extra-virgin olive oil turns out to go rancid at about the same rate in the cold as it does at room temperature. Chemists at the University of Udine in Italy found that the benefits of slowed reactions at low temperature are counterbalanced by the gradual crystallization of the more saturated oil molecules. This process leaves the remaining liquid oil with a higher proportion of vulnerable unsaturated fats, and with a smaller proportion of antioxidant substances, which tend to get trapped in the crystals. So oxidation continues at about the same pace despite the low temperature.

There's no need, then, to crowd the fridge with bottles of special olive oil. Just keep them in a cool, dark place. And this is especially true for unopened bottles. Another study from the Universities of Milan and Castilla-La Mancha reports that freshly bottled Italian and Spanish oils high in antioxidants retained much of their antioxidant capacity after as much as 240 days storage at 40 degrees Centigrade, or 104 degrees F. One oil made from the Picual olive even remained fully within the specifications for extra-virgin olive oil.

One other useful fact to be gleaned from the first report: the viscosity of olive oil nearly triples as it cools from room to refrigerator temperature. As many cooks know, you can make a plain vinaigrette very thick and creamy simply by serving it and the salad ice-cold.